Characterizing Current and Wave Drag over Shallow Coral Reefs

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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.