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Abstract 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.
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Loss of reef roughness increases residence time on an idealized coral reef
Abstract Through idealized, numerical models this paper investigates flows on a reef geometry which has received significant attention in the literature; a shallow, fringing reef with deeper, shore-ward pools or lagoons. Given identical model geometries and varying only reef flat drag coefficients between model runs ( $$C_D = [0.001,0.005,0.01,0.05,0.1]$$ C D = [ 0.001 , 0.005 , 0.01 , 0.05 , 0.1 ] ), two distinct circulation patterns emerge. One is related to low reef water levels and high roughness, and efficiently flushes the entire reef system resulting in low residence times (an ‘open reef’). The other is related to high reef water levels and low roughness, and in spite of the development of an offshore undertow, this dynamic is inefficient at flushing the reef-pool system and facilitating exchange flow with offshore waters (a ‘closed reef’). This paper shows that even given indistinguishable geometry and offshore conditions, this information is insufficient to predict reef dynamics, and suggests that reef roughness (and thus reef health) plays a comparable role in determining circulation patterns and residence times. Furthermore, a transition from open to closed or vice versa caused by e.g., a loss of reef roughness or increase in mean sea level could have implications for transport and mixing of nutrients and water masses, as well as larval dispersal.
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- Award ID(s):
- 1948189
- PAR ID:
- 10409391
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41598-022-24045-4
