Observations and prediction of intermittent sediment suspension in the surf zone Public Deposited

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  • In this dissertation the interactions between hydrodynamics, sediment suspension and transport, and morphological evolution in the surf zone was investigated with a large-scale laboratory experiment data, CROss-Shore Sediment Transport Experiment (CROSSTEX). The data set included comprehensive measurements of water surface elevation, fluid velocity, sediment concentration, and morphology for irregular waves under erosive and accretive beach conditions. First, hydrodynamics were examined in response to morphological evolution, focusing on turbulence due to wave breaking. For the erosive and accretive beach conditions, wave breaking characteristics, such as wave heights, average rate of energy dissipation by bores, and surf similarity parameter, were investigated in response to morphodynamics of the bar. Time-averaged turbulent kinetic energy was closely related to wave energy dissipation, supporting that wave energy dissipation is the main source of turbulent kinetic energy production in the surf zone. From this, it was found that wave energy, turbulent estimates, and morphodynamics in the surf zone were closely related to each other and they were quantitatively examined. Second, intermittent features of sediment suspension and turbulence, and their relationship were examined. 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. Also the relationship between the turbulence and sediment suspension events were explored with conditional probabilities. Here, 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 mechanism for the initiation of sediment suspension in the surf zone. It was also found that the intermittent sediment suspension events significantly contributed to onshore sediment transport. Finally, the intermittent sediment suspension was predicted with an artificial neural network. Input hydrodynamics consisted of low-frequency motions, wave-induced motions, and turbulent kinetic energy near the bed and near water surface level. Artificial neural network provided a prominent prediction capability of sediment suspension, showing a correlation coefficient up to 0.79 at the bar crest in the accretive beach condition. From the investigation of various combinations of input data, it was found that turbulence is the most contributing factor for sediment suspension in the surf zone. The inclusion of the information from the upper sensor increases the prediction at the bar trough in the erosive case. 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.
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