Abstract Coastal sediment transport of graded spherical particles with the material properties of sand was simulated and compared to transport of the uniform-sized spherical particles using a two-phase Eulerian-Lagrangian model. The fluid phase solver was based on SedFoam developed in OpenFOAM and the open-source discrete element method solver LIGGGHTS was used for the particle phase. We validated the model for sheet flow of well-sorted medium sand (d50 = 0.28 mm) and mixed sand with bimodal size distribution in velocity-skewed oscillatory flows as well as well-sorted coarse sand (d50 = 0.51 mm) in velocity-skewed and acceleration-skewed oscillatory flows. Simulation results of graded particles showed the formation of inverse grading (upward coarsening) in sediment bed under oscillatory flows, suggesting that the effects of armoring and exposure were important in the resulting transport rate. Examining different particle size distributions under onshore velocity-skewed flows, it was found that the largest increase of the net onshore sediment transport rate due to size gradation corresponded to the moderately sorted particle size distribution (d90/d10 = 3.41), where the coarse fraction (d > d50) had the maximum contribution to the transport. By analyzing intra-wave sediment transport quantities, the response of size gradation to the flow skewness and asymmetry and velocity intensity was investigated. Model results revealed that the armoring effect (reduction of sediment flux due to inverse grading) was dominant when flow velocity magnitude was lower (wave trough) or the fluid acceleration was higher. On the other hand, when flow velocity magnitude was larger (wave crest), the armoring effect was reduced or the exposure effect (enhancement of sediment flux due to inverse grading) may become more pronounced. Overall, we found that onshore net sediment transport was enhanced up to 30% due to particle size gradation under onshore velocity-skewed oscillatory flows. Conversely, the size gradation reduced the net onshore transport rate up to 35% under onshore acceleration-skewed oscillatory flows. Model results also suggested that the thickness of the active layer (surface layer affected by vertical sorting) in sheet flows can be quantified by the peak erosion depth. The simulation results presented here provide insights into the role of sediment size gradation (armoring and exposure effects) in wave-driven onshore transport which is important for predicting morphological evolution.

中文翻译：

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波浪驱动沉积物输运的装甲和暴露效应

摘要 使用两相欧拉-拉格朗日模型模拟了具有砂材料特性的分级球形颗粒的沿海沉积物输送，并与均匀尺寸的球形颗粒的输送进行了比较。流体相求解器基于OpenFOAM开发的SedFoam，粒子相使用开源离散元法求解器LIGGGHTS。我们验证了分选良好的中砂 (d50 = 0.28 mm) 和具有双峰粒度分布的混合砂在速度偏斜振荡流中以及在速度偏斜中分选良好的粗砂 (d50 = 0.51 mm) 的片状流模型和加速度偏斜的振荡流。分级颗粒的模拟结果表明，振荡流作用下沉积物床层形成逆分级（向上粗化），表明装甲和暴露的影响对最终的运输率很重要。检查陆上速度偏流下的不同粒径分布，发现由于粒径级配导致陆上沉积物净输运速率的最大增加对应于中等分选的粒径分布 (d90/d10 = 3.41)，其中粗粒级(d > d50) 对运输的贡献最大。通过分析波浪内泥沙输运量，研究了粒度级配对流动偏度和不对称性以及速度强度的响应。模型结果表明，当流速幅度较低（波谷）或流体加速度较高时，装甲效应（由于逆级配而减少沉积物通量）占主导地位。另一方面，当流速幅度较大（波峰）时，装甲效应减弱或暴露效应（由于逆级配而增加沉积物通量）可能变得更加明显。总体而言，我们发现陆上净沉积物输送由于陆上速度偏斜振荡流下的粒度级配而增强了 30%。相反，在陆上加速度偏斜的振荡流下，尺寸等级将陆上净运输率降低了 35%。模型结果还表明，片流中活动层（受垂直分选影响的表层）的厚度可以通过峰值侵蚀深度来量化。