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Abstract: A predictive model of time-dependent sediment suspension was developed with artificial neural network, using velocity and integrated sediment concentration (i.e., suspended load) data observed from a large-scale laboratory experiment under erosive and accretive beach conditions. Low-frequency motion, wave-induced motion, and turbulent kinetic energy per unit mass at lower (1cm above the bed) and upper (50cm above the bed) ADV were used as inputs and suspended load near the bed was used as output. To account for the history prior to sediment suspension, five time steps (corresponding to 2s) of data prior to sediment suspension were included in the inputs. The artificial neural network model shows a predictive capability of the time-dependent sediment suspension in the surf zone for both erosive and accretive conditions. The correlation coefficient (r) between the measured and predicted suspended load reached up to r =0.79 at the bar crest in the accretive beach condition, and typical average correlation coefficients over all subsets (r̄) were 0.53 over the bar crest. The results from various input cases showed that turbulent kinetic energy near the bed is most important to predict the sediment suspension in the surf zone for both erosive and accretive conditions. Also, the best predictive performance was shown at the bar crest, supporting the hypothesis that turbulence induced by intense wave breaking at the bar crest is a dominant mechanism for sediment suspension. Outside the surf zone, the wave-induced motion was the highest contributing factor for the sediment suspension. Furthermore, when the information from the upper ADV was included as inputs, the model improved the prediction capability by 40% at the bar trough in the erosive beach condition. The increased prediction at this location may possibly be attributed to the effect of vertical shear motion in the low-frequency range due to strong undertow from wave breaking. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Laboratory measurements of the instantaneous free surface, horizontal velocity, and void fraction fluctuations were made simultaneously for three cases of regular waves breaking on a plane slope. The data were reduced by ensemble averaging to quantify the temporal variation of the turbulence intensity and void fraction above trough level in the aeration region of the breaking waves. The cross-shore location of the measurements was restricted to the transition region marked by a rapid decrease in wave height. The study showed that the maximum ensemble-averaged void fractions were between 15 and 20% and that the temporal variation of the normalized void fraction above the still water level could be modeled by linear growth followed by exponential decay. The temporal variation of void fraction above the still water normalized by the wave period and average void fraction appears to be self-similar. [ABSTRACT FROM AUTHOR]

The large-scale structure of intermittent turbulence below trough level produced by depthlimited wave breaking was investigated in a laboratory wave flume using point velocity measurements and particle image velocimetry. A conditional sampling technique was applied to the instantaneous turbulent kinetic energy per unit mass, k/ρ, estimated from the point velocity measurements where the large scale turbulent fluctuations were separated from the orbital wave motion by phase-averaging. The analysis shows that large, intermittent events exceeding a threshold equal to the mean and standard deviation of kip occur for only 7% of the record but account for approximately 40% of the total turbulent kinetic energy below trough level. The PIV technique was used to quantify the size and vorticity of the large eddies associated with wave breaking. The PIV technique was applied such that the light sheet was in the horizontal plane to quantify the eddy size and vorticity. Observations show that the nominal diameter of the eddy was 0.05 m with a maximum vorticity of 30 1/s for laboratory scale wave breaking. [ABSTRACT FROM AUTHOR]