Abstract
Free surface elevation time series of breaking water waves were measured in a laboratory flume. This was done in order to analyze changes in wave characteristics as the waves propagated from deep water to the shore. A pair of parallel- wire capacitive wave gages was used to simultaneously measure free surface elevations at different positions along the flume. One gage was kept fixed near the wave generator to provide a reference while the other was moved in steps of 0.1 m in the vicinity of the break point. Data from these two wave gages measured at the same time constitute station-to-station free surface elevation time series. Fast Fourier Transform (FFT) based cross-correlation techniques were employed to determine the time lag between each pair of the time series. The time lag was used to compute the phase shift between the reference wave gage and that at various points along the flume. Phase differences between two points spaced 0.1 m apart were used to calculate local mean wave phase velocity for a point that lies in the middle. Results show that moving from deep water to shallow water, the measured mean phase velocity decreases almost linearly from about 1.75 m/s to about 1.50 m/s at the break point. Just after the break point, wave phase velocity abruptly increases to a maximum value of 1.87 m/s observed at a position 30 cm downstream of the break point. Thereafter, the phase velocity decreases, reaching a minimum of about 1.30 m/s.
Highlights
IntroductionBeginning offshore where the water depth is sufficiently deep and constant, water waves are observed to be symmetric with respect to the wave crest before they begin to deform due to interactions with the bathymetry [3]
Laboratory experiments were perofrmed in wave flume to determine the accuracy of small-amplitude wave theory in predicting the transformation of monochromatic two-dimensional waves as they propagated from intermediate to shallow water depths
Local wave phase velocity was calculated for points 0.1 m apart
Summary
Beginning offshore where the water depth is sufficiently deep and constant, water waves are observed to be symmetric with respect to the wave crest before they begin to deform due to interactions with the bathymetry [3] As they propagate from deep water to shallow water of the surf zone beach slope, they slow down and grow taller. At a depth of half its wave length, rounded waves start to rise and their crests become shorter while their troughs lengthen Their period (frequency) stays the same, the waves slow down and their overall wavelength shortens. Breaking of waves is characterized by top of the crest falling onto the front face of the wave, forming a body of fluid, called the roller that rides on the wave front This process entraps considerable amount of air which bursts into small bubbles, and results in energy dissipation and the transfer of momentum to currents. There will be a shoreward mass transport occurring above the trough level
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