A Mechanistic Model for Mud Flocculation in Freshwater Rivers
A Mechanistic Model for Mud Flocculation in Freshwater Rivers
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DOI：
10.1029/2021jf006392
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发表时间：
2022-05
期刊：
影响因子：
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通讯作者：
J. Nghiem;W. Fischer;Gen K. Li;M. Lamb
中科院分区：
文献类型：
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作者：
J. Nghiem;W. Fischer;Gen K. Li;M. Lamb
泥沙在河流中的迁移和沉积是河流地貌和生物地球化学循环中的关键过程。最近的研究表明，絮凝作用可能通过提高泥沙的沉降速度来调节河流泥沙的运移，但我们还缺乏淡水河流中絮凝作用的定标机理模型。在这里，我们开发并校准了一个关于河流中絮体直径和沉降速度的半经验模型。我们编制了全球河流悬浮泥沙浓度-深度剖面数据集，并利用Rouse-Vanoni方程对其进行了原位沉降速度的反演。平均而言，黏土和粉土(直径39μm)被絮凝，沉降速度为1.8mmKolmogorov−1，絮体直径为130μm。在模型变量中，柯尔莫戈罗夫微尺度与絮体直径呈最强的正相关，支持湍流剪切限制絮体尺寸的观点。沉积物Al/Si(矿物学指标)与絮体直径和沉降速度呈最强的负相关，说明粘土丰度和组成对絮凝的重要性。絮凝体的沉降速度随着泥浆和有机质浓度的增加而增加，这与颗粒碰撞驱动的絮凝和通常集中在泥浆中的有机物的结合是一致的。相对电荷密度(盐度指标)与较小的絮体沉降速度相关，这一发现可能反映了有机物的原始粒度分布和物理赋存状态。校正后的模型对河流絮体沉降速度数据的解释系数约为两倍。结果表明，絮凝作用可以影响泥浆和颗粒有机碳的命运，对全球生物地球化学循环具有影响。
The transport and deposition of mud in rivers are key processes in fluvial geomorphology and biogeochemical cycles. Recent work indicates that flocculation might regulate fluvial mud transport by increasing mud settling velocities, but we lack a calibrated mechanistic model for flocculation in freshwater rivers. Here, we developed and calibrated a semi‐empirical model for floc diameter and settling velocity in rivers. We compiled a global data set of river suspended sediment concentration‐depth profiles and inverted them for in situ settling velocity using the Rouse‐Vanoni equation. On average, clay and silt (diameters <39 μm) are flocculated with settling velocity of 1.8 mm s−1 and floc diameter of 130 μm. Among model variables, Kolmogorov microscale has the strongest positive correlation with floc diameter, supporting the idea that turbulent shear limits floc size. Sediment Al/Si (a mineralogy proxy) has the strongest negative correlation with floc diameter and settling velocity, indicating the importance of clay abundance and composition for flocculation. Floc settling velocity increases with greater mud and organic matter concentrations, consistent with flocculation driven by particle collisions and binding by organic matter which is often concentrated in mud. Relative charge density (a salinity proxy) correlates with smaller floc settling velocities, a finding that might reflect the primary particle size distribution and physical hosting of organic matter. The calibrated model explains river floc settling velocity data within a factor of about two. Results highlight that flocculation can impact the fate of mud and particulate organic carbon, holding implications for global biogeochemical cycles.