Cohesive sediment transport, where its settling velocity is controlled by the flocculation process, is a crucial component in determining biochemical cycles, fate of pollutants, and morphodynamics in many aquatic ecosystems. In this study, a modeling framework is presented to investigate how flocculation influences cohesive sediment transport in the current bottom boundary layer in dilute conditions, consistent with the calibration range of the flocculation model. From a local analysis of floc dynamics in homogenous turbulence, we identify that the floc size distribution is mainly controlled by floc cohesion and yield strength. The uncertainty in fractal dimension plays a minor role for the floc size but it influences the resulting floc density and settling velocity. The transport analysis in the current boundary layer shows that the flocculation process alters the vertical distribution of the settling velocity and hence the sediment concentration with a strong dependence on cohesion, floc yield strength, and floc structure. When the flocs are more susceptible to breaking, a well-mixed concentration profile is obtained. In contrast, for flocs with higher cohesion or yield strength, higher concentration with a sharp gradient is observed close to the bed. Overall, the settling velocity exhibits a low vertical variability within 20 % of the depth-averaged value except near the bed. This suggests that using a depth-averaged settling velocity yields acceptable predictions of the sediment concentration profiles, especially for flocs with lower cohesion.

中文翻译：

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当前底部边界层中絮凝粘性沉积物输运的建模框架


粘性沉积物输送，其沉降速度由絮凝过程控制，是决定许多水生生态系统中生化循环、污染物归宿和形态动力学的关键组成部分。在本研究中，提出了一个建模框架，以研究在稀释条件下絮凝如何影响当前底部边界层中的粘性沉积物传输，这与絮凝模型的校准范围一致。通过对均质湍流中絮体动力学的局部分析，我们确定絮体尺寸分布主要受絮体内聚力和屈服强度控制。分形维数的不确定性对絮凝体大小起着次要作用，但它会影响得到的絮凝体密度和沉降速度。当前边界层的输运分析表明，絮凝过程改变了沉降速度的垂直分布，从而改变了沉积物浓度，与内聚力、絮凝体屈服强度和絮凝体结构有很大关系。当絮凝体更容易破裂时，可获得充分混合的浓度曲线。相比之下，对于具有较高内聚力或屈服强度的絮凝体，在靠近床层的地方观察到更高的浓度和陡峭的梯度。总体而言，沉降速度表现出较低的垂直变化，误差在深度平均值的 20% 以内，但靠近床层时除外。这表明，使用深度平均沉降速度可以产生可接受的沉积物浓度分布预测，特别是对于内聚力较低的絮凝体。