Heterogeneous effects on the flow structure near a coral-colony canopy layer

• Main Authors: Jie Yang, 楊杰
• Other Authors: Zhi-Cheng Huang
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/37191372461361277569

碩士 === 國立中央大學 === 水文與海洋科學研究所 === 104 === The north and south channels in Dongsha Atoll National Park play an important role in biological ecosystems and biogeochemical and biological status process. The boundary flow near the canopy affects micro-scale coral reef ecosystem, for example, the larval transport behaviors and organism distribution. Here, we present a four-week in-situ measurement of waves and currents. The dataset covers two spring-neap cycles that the major solar tide modulated the major lunar tide. The near-bottom current profiles were measured by one bottom mounted pulse-coherent acoustic Doppler profiler and the turbulent flow motions were measured by two acoustic Doppler velocimeters. The results show that (1) the slope of the vertical current profile will change with the distribution of the canopy, the height of inflection point of slope is comparable to the height of the coral bommies. (2)Assuming that the flow above the canopy follow the law-of-the-wall, the friction velocity u* can be calculate from the current profile. We find that the friction velocity and the hydraulic roughness during the flood tide period is greater than that of ebb tide, and there is a positive correlation between u* and Uc, and negative correlation between z0 and Uc. This result could be due to the flow interacting with different colony structures. During flood tide events, the water flow passes through dense coral colonies, which produce the significant shear stress. (3)The flow structure in the canopy layer is affected by the wake produced by coral colonies. The velocity inside the canopy is shown to decrease as reef frontal area is increased. A attenuation parameter αc is used to quantify the reduction of the in-canopy velocity. The result shows that the values of αc during ebb tides are 1.5 times higher than that during the flood tides, indicating a low velocity in-canopy flow during flood tide. This implies an enhanced in-canopy mixing and larger form drag caused by the dense coral colonies.