Hydrodynamics of Coral Reefs

The geometric complexity of coral reefs leads to interesting fluid mechanics problems at scales ranging from those of coral colonies or even branches a few millimeters in diameter up to whole reefs that can be kilometers in horizontal extent. In many cases, both at the colony and reef scale, unsteady flows, usually due to surface waves, behave very differently than do steady flows for which the coral structures may appear to have quite high resistance to any flow through their interior. Allowing for this difference, engineering formulae for mass transfer describe well the uptake of nutrients by corals, although a priori determination of hydrodynamic roughness of corals and coral reefs is not yet possible. Surface wave-driven flows are a common feature of many coral reefs and appear to follow predictions of theories based on radiation stress gradients. However, comparisons to observations have been relatively limited, and there is some question as to the role played by Stokes drift in these flows. Like other near-shore environments, internal waves and flows driven by horizontal buoyancy gradients can also be important.