Wave Modulation of Flows on Open and Closed Reefs

Using observations, numerical models, and theory, we explore a framework to classify reefs as open or closed based on their dynamics. While the concepts of open and closed reefs are used widely in studies of coral reef hydrodynamics and are generally based on geometry, there is no consensus on what qualifies as open and closed. With observations from Ofu, American Samoa, we show that the reef flat exhibits two different dynamical regimes depending on tidal and wave forcing. Flow over this reef flat resembles a classic one‐dimensional barrier reef flow during low tide, where wave setup creates a cross‐reef pressure gradient which forces flow on the flat. On high tide, however, flow on the flat is oblique to the crest, and at times directed offshore. We reproduce this behavior in an idealized numerical model of a fringing reef. We classify open reefs as a condition where an onshore, wave‐generated pressure gradient is balanced by friction, and closed reefs as a condition where an onshore radiation stress gradient is opposed by an offshore pressure gradient. Results from the fringing reef model show that the system transitions between open and closed behavior over a tidal cycle. Results from an additional barrier reef numerical model exhibits almost exclusively open reef behavior, for which we derive a simple theoretical model. We argue that classifying reefs as open or closed based on their dynamics, rather than geometry, is a more meaningful approach to comparing reefs and predicting their dynamical response to wave and tidal forcing.