Abstract In shallow water atop many isolated platforms and atolls, reef sand aprons largely consist of debris shed platformward from shelf-margin reefs towards the lagoon. Although their general geomorphology and sedimentology is broadly understood, quantitative details of the possible role of reef sand apron hydrodynamics on their geomorphic evolution remain less well constrained. To test the hypothesis that on-platform reef sand apron progradation is prone to completely infill adjacent lagoons, this study documents over 50 new hydrodynamic simulations that isolate and evaluate the relations among geomorphology, waves, and tides. The results show how deep-water waves hit the shelf margin and favor on-platform sediment transport, but highlight that tides modulate the influence of waves while generating currents on their own. Across the range of wave heights and tidal amplitudes, however, platform-directed bed shear stress on shallow reef flats decreases with increasing reef and reef sand apron width. Parts of broad reef sand aprons can even include off-platform-directed shear stress during incoming flood tide. These insights motivate a conceptual model for how the process of widening of a reef sand apron by lagoonward sediment transport decreases the magnitude of the very forces that drive the transport, inhibiting further progradation. Such an autogenic, self-limiting dynamic curbs the propensity of lagoons to fill with coarse sediment shed from the reef by reef sand apron expansion. Instead, many atolls are doomed to remain “half-empty buckets,” even in the absence of external change such as a relative change in sea level.

中文翻译：

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平台顶礁砂围裙形态动力学和半空桶

摘要 在许多孤立平台和环礁顶部的浅水区，礁砂围裙主要由从陆架边缘礁向泻湖向平台脱落的碎屑组成。尽管它们的一般地貌和沉积学已被广泛理解，但关于礁砂围裙流体动力学对其地貌演化可能起的作用的定量细节仍然没有受到很好的限制。为了验证平台上珊瑚礁砂裙板进积容易完全填满相邻泻湖的假设，本研究记录了 50 多个新的水动力模拟，这些模拟分离和评估了地貌、波浪和潮汐之间的关系。结果显示深水波浪如何撞击陆架边缘并有利于平台上的沉积物运输，但强调潮汐调节波浪的影响，同时自身产生电流。然而，在整个波高和潮汐幅度范围内，浅礁滩上的平台导向床剪应力随着礁石和礁砂裙板宽度的增加而减小。宽阔的礁砂围裙的一部分甚至可以在来潮期间包括平台外定向的剪切应力。这些见解激发了一个概念模型，即通过向泻湖的沉积物运输扩大珊瑚礁沙裙的过程如何降低驱动运输的力量的大小，从而抑制进一步的进积。这种自生的、自限性的动态抑制了泻湖填充因礁砂围裙膨胀而从礁石脱落的粗沉积物的倾向。相反，即使没有外部变化（例如海平面的相对变化），许多环礁也注定要保持“半空桶”。浅礁滩的平台导向床剪应力随着礁石和礁砂裙板宽度的增加而减小。宽阔的礁砂围裙的一部分甚至可以在来潮期间包括平台外定向的剪切应力。这些见解激发了一个概念模型，即通过向泻湖的沉积物运输扩大珊瑚礁沙裙的过程如何降低驱动运输的力量的大小，从而抑制进一步的进积。这种自生的、自限性的动态抑制了泻湖填充因礁砂围裙膨胀而从礁石脱落的粗沉积物的倾向。相反，即使没有外部变化（例如海平面的相对变化），许多环礁也注定要保持“半空桶”。浅礁滩的平台导向床剪应力随着礁石和礁砂裙板宽度的增加而减小。宽阔的礁砂围裙的一部分甚至可以在来潮期间包括平台外定向的剪切应力。这些见解激发了一个概念模型，即通过向泻湖的沉积物运输扩大珊瑚礁沙裙的过程如何降低驱动运输的力量的大小，从而抑制进一步的进积。这种自生的、自限性的动态抑制了泻湖填充因礁砂围裙膨胀而从礁石脱落的粗沉积物的倾向。相反，即使没有外部变化（例如海平面的相对变化），许多环礁也注定要保持“半空桶”。