Characterizing Current and Wave Drag over Shallow Coral Reefs

珊瑚礁的水动力学特征是粗糙的。因此，在理解和模拟浅珊瑚礁上的海流和表面重力波的动力学的基本挑战是表征阻力。这项研究旨在为珊瑚礁开发更准确的环流和波浪模型，这对于正确解释珊瑚礁的观测和模拟生物化学过程至关重要。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至关重要。例如，拖曳系数对水深的依赖性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，从而直接有益于正在进行的关于海洋酸化对帕劳（热带太平洋西部）和东沙（中国南海）堡礁影响的研究。该项目将为两名研究生的论文工作提供信息。为了促进对珊瑚礁的水流和波浪阻力的理解，本研究解决了四组问题：1）基于深度平均水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，这种依赖性是否解释了珊瑚礁阻力系数的分散？2)珊瑚礁上方的表面重力波摩擦系数与轨道位移之间的关系是什么，它是否与实验室研究结果和现有理论相一致？3)波流相互作用是否显著地增强了珊瑚礁上方的水流阻力和波浪耗散？4)水流和表面波的流体动力学粗糙度估算是否相似？它们与珊瑚礁上的物理粗糙度有什么关系？ 这四组问题将利用现有的观测资料，目前的配置文件，压力和波的特点，从三个非常不同的珊瑚礁系统，帕劳的堡礁，东沙环礁，并在红海的三个平台礁。空间平均阻力系数的估算方法是，对珊瑚礁的动量平衡进行积分，以考虑到水深的空间（和时间）变化，并确定使动量平衡的残余误差最小的阻力系数。将通过整合整个珊瑚礁的波能平衡和最大限度地减少残余误差来估计波浪摩擦系数。 由于珊瑚礁上方的水深在空间和时间上有很大的变化，确定拖曳系数对水深的依赖关系并提供这种依赖关系的模型将是对珊瑚礁流体动力学的理解的一个重大进展。该项目将促进对珊瑚礁上的波流相互作用的了解，这是一个潜在的重要的未解决问题。考虑到这些影响海流和波浪阻力的因素，可以准确和一致地估计水动力粗糙度，然后可以与珊瑚礁的物理粗糙度。

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.