東沙環礁海洋國家公園的南北水道對大潟湖生物、生態以及生地化過程扮演重要的角色。在靠近底床珊瑚礁叢集附近的邊界層流動特性對細部的珊瑚礁生物系統有相當大的影響，例如珊瑚幼子或有機物之傳輸擴散。本實驗在東沙環礁之西北水道進行了長達四週對波浪、海流之觀測，觀測期間共包含了兩個大小潮週期之資料。單點高頻之流速資料由兩支聲學都普勒流速儀(ADVO)觀測，另外，流速剖面由脈衝相干式剖面流速儀(PC-ADP)量測。本次研究分別針對珊瑚礁冠層以上流場以及珊瑚礁冠層之內的流場進行分析。本研究結果發現，(1)在珊瑚礁冠層附近之垂直平均流的剖面斜率依漲退潮會有所差異，斜率之變化發生在珊瑚礁叢集高度的位置。(2)若假設珊瑚礁冠層之上可適用邊壁理論(law of the wall)，並依此推估摩擦速度u*。結果發現，漲潮時所觀測到之摩擦速度會大於退潮時段所觀測到之流場摩擦速度，且其值有隨流速的增加而增大的趨勢。我們推測此摩擦速度隨流向改變的現象應該是因潮流經過不同密度的珊瑚礁所導致。在漲潮時段，潮水流經大片而密集之珊瑚礁叢集，粗糙礁體提供之形狀阻力(from drag)使得局部點位所觀測到之摩擦速度增加；反之，退潮之海水流經分佈較稀疏的珊瑚礁叢集，故所觀測到之摩擦速度較小。(3)在珊瑚礁冠層以下，流場會因為礁體所形成之尾流效應而產生變化，其流速會隨海水所流經之珊瑚礁截面積(frontal area)增大而降低。我們利用了流速衰減參數(attenuation parameter,αc)來探討珊瑚礁冠以層內流速與外部自由流流速的衰減特性。結果顯示，退潮時段之流速衰減參數為漲潮時段之流速衰減參數的1.5倍。此現象亦可由珊瑚礁體之分佈來解釋，漲潮時段流向所對應之珊瑚礁截面積較大，導致珊瑚礁冠層以下之流速大幅衰減；退潮時段流向所流經之礁體截面積較小，所以流速之衰減率較小。;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.
