Near-Shore Sediment Transport Under Cnoidal Waves Using Particle Image Velocimetry
Katrina, Jenna Rose
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With future trends showing a higher percentage of the human population living along the coast, coastal resilience is a significant topic of concern for many researchers. The purpose of this research was to analyze near-shore sediment transport under different wave parameters using the technique of Particle Image Velocimetry (PIV) and laboratory studies for linear and nonlinear cnoidal waves. At the deeper extent of the nearshore profile, waves will tend to be linear, having an Ursell number less than 40, with little or no net sediment transport due to orbital velocities. The velocities are the same under the crest and trough and will be dominated by undertow for linear waves. It is hypothesized that as the wave propagates shoreward, the wave transforms into a nonlinear cnoidal wave and the skewness of a cnoidal wave profile, having an Ursell number greater than 40, will drive nearshore sediment transport onshore. The project goal was to quantify the volumetric sediment transport rates based on these dimensionless parameters and analyze how the linearity of waves affect the sediment transport onshore and offshore. To accomplish this goal, a sandy bed with a sediment distribution mimicking that found on East Coast beaches, was constructed in the Florida Tech wave channel. A range of waves were generated at three different water depths and three different frequencies over a flat and rippled bed. The velocity near the bed was analyzed in the laboratory using a PIV system consisting of a laser to illuminate the sediment particles and a high-frame rate camera to record videos, which were analyzed in the Matlab PIV toolbox, PIVlab. Through image analysis, a relationship was developed between wave characteristics and dimensionless parameters, the Ursell number and Shields parameter, wave linearity, and sediment movement. Furthermore, time-lapse videography of the sediment movement was recorded and analyzed to determine accretion/erosion rates caused by these conditions. The time-lapse video, paired with the results from the PIV toolbox, was used to quantify sediment movement. The results from this study elucidated the relationship between water depth, wave height, wave period, and wavelength to the linearity of the wave, the velocity of the sediment being transported and the corresponding Ursell and Shields parameter. The results will aid in the development of improved models for predicting bed morphology, which will lead to a better understanding of coastal resilience in the future.