Wave Modulation of Flows on Open and Closed Reefs
Wave Modulation of Flows on Open and Closed Reefs
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DOI：
10.1029/2020jc016645
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发表时间：
2021-04-01
影响因子：
3.6
通讯作者：
Monismith, S. G.
中科院分区：
文献类型：
--
作者：
Lindhart, M.;Rogers, J. S.;Monismith, S. G.
利用观测、数值模型和理论，我们探索了一种根据珊瑚礁的动力学将其分类为开放或关闭的框架。虽然开放珊瑚礁和闭合珊瑚礁的概念在珊瑚礁水动力学研究中被广泛使用，并且通常是基于几何学，但对于什么是开放和闭合，没有达成共识。通过对美属萨摩亚Ofu的观测，我们发现，在潮汐和波浪强迫的作用下，礁坪呈现出两种不同的动力机制。在这个礁滩上的流动类似于退潮时经典的一维堰塞礁流动，在这种情况下，波的设置产生了一个跨礁压力梯度，迫使水流在平地上流动。然而，在涨潮时，平地上的水流与顶端倾斜，有时流向近海。我们在一个理想化的珊瑚礁数值模型中再现了这种行为。我们将开放珊瑚礁归类为岸上波浪产生的压力梯度通过摩擦平衡的条件，将封闭珊瑚礁归类为岸上辐射应力梯度与离岸压力梯度相反的条件。边缘礁模型的结果表明，在一个潮汐周期内，系统在打开和关闭行为之间转换。一个额外的障碍礁数值模型的结果显示了几乎完全开放的礁石行为，我们得到了一个简单的理论模型。我们认为，根据珊瑚礁的动力学而不是几何形状将珊瑚礁分类为开放或封闭是一种更有意义的方法，可以比较珊瑚礁并预测其对波浪和潮汐强迫的动力学响应。
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.