Characterizing Current and Wave Drag over Shallow Coral Reefs

珊瑚礁最重要的水动力特征就是它们是粗糙的。因此，对浅层珊瑚礁上的海流和表面重力波的动力学进行理解和建模的根本挑战是确定阻力的特征。这项研究的目的是开发更准确的珊瑚礁环流和波浪模型，这对于正确解释观测结果和模拟珊瑚礁上的生化过程至关重要。更好地了解水动力学对于准确评估气候变化对珊瑚礁的影响也是至关重要的。例如，阻力系数对水深的依赖显然对海平面上升对珊瑚礁的影响有影响。这项研究将为解释生物化学观测提供更好的物理背景，从而直接有利于正在进行的关于海洋酸化对帕劳(西热带太平洋)和东沙(南中国海)堡礁影响的研究。该项目将指导两名研究生的毕业论文工作。为了促进对珊瑚礁上的海流和海浪阻力的理解，这项研究解决了四组问题：1)基于深度平均流的阻力系数是否以与明渠水流理论一致的方式依赖于水深，这种依赖关系是否解释了珊瑚礁阻力系数的部分分散？2)珊瑚礁表面重力波摩擦系数与轨道位移之间的关系是什么，它与实验室研究和现有理论的结果是否一致？3)波-流相互作用是否显著增强了珊瑚礁上的水流阻力和波浪消散？4)流和表面波的水动力粗糙度估计是否相似，它们与珊瑚礁上的物理粗糙度之间的关系是什么？这四组问题将通过对三个非常不同的珊瑚礁系统--帕劳的堰塞礁、东沙环礁和红海的三个平台礁--的当前剖面、压力和波浪特征的现有观测来解决。空间平均阻力系数将通过对珊瑚礁上的动量平衡进行积分，以考虑水深的空间(和时间)变化，并确定使动量平衡中的残余误差最小的阻力系数来估计。波浪摩擦系数将通过对整个礁体的波能平衡进行积分并使残差最小化来估计。由于珊瑚礁上的水深在空间和时间上都有很大的变化，因此确定阻力系数与水深的关系并为这种关系提供一个模型将是对珊瑚礁水动力学理解的一个重大进步。该项目将促进对珊瑚礁上波-流相互作用的理解，这是一个潜在的重要悬而未决的问题。考虑到这些影响水流和波浪阻力的因素，将能够准确和一致地估计水动力粗糙度，然后将其与珊瑚礁的物理粗糙度联系起来。

The defining hydrodynamic characteristic of coral reefs is that they are rough. Consequently, the fundamental challenge in understanding and modeling the dynamics of both currents and surface gravity waves over shallow coral reefs is characterizing drag. This research is aimed at developing more accurate circulation and wave models for coral reefs that are essential for properly interpreting observations and modeling biochemical processes over reefs. A better understanding of the hydrodynamics is also essential for making accurate assessments of the impact of climate change on coral reefs. For example, the dependence of drag coefficients on water depth clearly has implications to the impact of sea level rise on coral reefs. This study will directly benefit ongoing research on the impact of ocean acidification on the barrier reefs of Palau (western tropical Pacific) and Dongsha (South China Sea) by providing a better physical context for interpreting biochemical observations. The project will inform the thesis work of two graduate students. To advance the understanding of current and wave drag over coral reefs, this study addresses four sets of questions: 1) Do drag coefficients based on depth-average currents depend on water depth in a manner consistent with theory from open-channel flow and does this dependence account for some of the scatter in coral reef drag coefficients? 2) What is the relationship between surface gravity wave friction factors over coral reefs and orbital displacements and is it consistent with results from laboratory studies and existing theories? 3) Do wave-current interactions significantly enhance current drag and wave dissipation over coral reefs? 4) Are hydrodynamic roughness estimates for currents and surface waves similar and what is their relationship to physical roughness over coral reefs? These four sets of questions will be addressed use existing observations of current profiles, pressures, and wave characteristics from three very different coral reef systems, Palau's barrier reef, Dongsha Atoll, and three platform reefs in the Red Sea. Spatial-average drag coefficients will be estimated by integrating the momentum balance across the reefs to account for spatial (and temporal) variations in water depth and determining the drag coefficient that minimizes the residual error in the momentum balance. Wave friction factors will be estimated by integrating the wave-energy balance across the reef and minimizing the residual error. Since there are substantial spatial and temporal variations in water depth over coral reefs determining the drag coefficient dependence on water depth and providing a model for that dependence will be a significant advance in the understanding of coral reef hydrodynamics. The project will advance the understanding of wave-current interactions over coral reefs, a potentially important unresolved problem. Accounting for these factors influencing current and wave drag will allow accurate and consistent estimates of hydrodynamic roughness that can then be related to coral reef physical roughness.