Abstract Oysters are ecosystem engineers that form reefs with rough surfaces. The complex surface of a reference-condition reef contains clusters of live oysters that protrude into and interact with flows. A degraded oyster reef contains fewer live oyster clusters and subsequently has different bed roughness. Such morphological differences may translate to varied hydrodynamic function, with influence to local and larger-scale flow fields and sediment transport, as well as larval recruitment and long-term reef sustainability. The objective of this study is to compare a degraded and a reference-condition intertidal oyster reef, and assess how differences in structure (reef morphology and roughness) influence near-bed hydrodynamic function within the roughness sublayer defined by oysters. Sediment entrainment and bed deformation were observed only on the reference oyster reef, despite that mean velocities were lower than those at the degraded reef. This was attributed to slightly finer bed material and higher turbulent kinetic energy, k, at the reference reef (k = 29.2 ± 4.5 cm2/s2 with mean velocity 11.5 ± 0.6 cm/s at 1 cm above the bed) compared to the degraded reef (k = 22.6 ± 4.2 cm2/s2 with mean velocity 16.1 ± 1.7 cm/s at 1 cm above the bed). At 1 cm above the bed, shear turbulence production at the degraded reef was significantly larger than turbulence dissipation rate, while at the reference reef this pattern was observed only for time periods with low turbulent kinetic energy. For measurements 5 cm above the bed at the reference reef, shear turbulence production and turbulence dissipation rates exhibited a relatively balanced pattern, with dissipation exceeding shear production for the time period with elevated turbulence. Four bed shear stress calculation methodologies converged over the degraded oyster reef but deviated considerably at the reference oyster reef. The drag coefficient associated with bed shear stress at the reference reef was almost two times greater than that at the degraded reef and almost an order of magnitude greater than those observed at sand and mud beds.

中文翻译：

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河口泻湖潮间带自然和退化牡蛎礁湍流的原位测量

摘要 牡蛎是生态系统工程师，它们形成表面粗糙的珊瑚礁。参考条件珊瑚礁的复杂表面包含成群的活牡蛎，它们伸入水流并与水流相互作用。退化的牡蛎礁含有较少的活牡蛎簇，因此具有不同的床粗糙度。这种形态差异可能转化为不同的水动力功能，影响局部和更大规模的流场和沉积物运输，以及幼虫补充和珊瑚礁的长期可持续性。本研究的目的是比较退​​化和参考条件的潮间带牡蛎礁，并评估结构差异（珊瑚礁形态和粗糙度）如何影响牡蛎定义的粗糙亚层内的近床水动力功能。尽管平均速度低于退化的礁石，但仅在参考牡蛎礁上观察到沉积物夹带和床变形。这归因于与退化的珊瑚礁相比，参考珊瑚礁（k = 29.2 ± 4.5 cm2/s2，平均速度为 11.5 ± 0.6 cm/s，在床上方 1 cm 处）略细的床物质和更高的湍流动能 k （k = 22.6 ± 4.2 cm2/s2，床上方 1 cm 处的平均速度为 16.1 ± 1.7 cm/s）。在床上方 1 cm 处，退化珊瑚礁的剪切湍流产生明显大于湍流耗散率，而在参考珊瑚礁，这种模式仅在湍动能低的时间段观察到。对于参考礁石床上方 5 cm 处的测量，剪切湍流产生和湍流耗散率表现出相对平衡的模式，在湍流升高的时间段内耗散超过剪切产生。四种床剪应力计算方法在退化的牡蛎礁上收敛，但在参考牡蛎礁上有很大偏差。参考生物礁与床层剪切应力相关的阻力系数几乎是退化生物礁的两倍，比在沙泥床中观察到的阻力系数大几乎一个数量级。