||This study aims to investigate the characteristics of wave attenuation over an algal reef, Guanyin village, Taoyuan, northwest coast of Taiwan. When ocean waves propagate from offshore into nearshore regions, the wave energy is reduced due to wave-breaking and bottom friction. In order to understand the physical mechanism of wave attenuation, we used acoustic Doppler velocimeter(ADV), ultrasonic Anemometer and camera to measure the displacement of water surface elevation, 3-D fluid velocity, 3-D wind velocity, and surface signatures due to wave-breaking in the field. The wave height and wave energy dissipation was determined from measured surface elevation data by using the Fourier spectral analysis combining with the linear wave theory. We used camera to record the water surface to determine the wave-breaking index and then to compare with the results reported by previous studies. The data were separated into two groups to separate the effects due to wave-breaking and bottom friction. For the wave-breaking case, we found that the wave energy is mainly dissipated near the dominant wave frequency of the incident waves. In addition, for the non-breaking case, the bottom friction is assumed the major contribution of wave dissipation; we compute the near-bottom velocity and use cubic regression to obtain the friction coefficient. The coefficient is found to be smaller than the recently reported value over coral reefs but is comparable to that over an algal reef found by Hsu . Two semiempirical models are used to estimate wave-breaking dissipation rate, including of Battjes and Janssen and Thornton and Guza , and the results show that wave-breaking dissipation rates predicted by the two models are higher than experimental results.|
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