Sheet flow dynamics under monochromatic nonbreaking waves
Sheet flow dynamics under monochromatic nonbreaking waves
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DOI：
10.1029/2001jc001045
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发表时间：
2002-10
影响因子：
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通讯作者：
Marjolein Dohmen-Janssen;D. Hanes
中科院分区：
文献类型：
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作者：
Marjolein Dohmen-Janssen;D. Hanes
首次在原型表面重力波作用下，结合对片状流上方悬浮过程的测量，对片状流内部的泥沙浓度和颗粒速度进行了详细的测量。用天然沙在大型波浪水槽中进行了试验。在大浪作用下的浅水输沙主要包含在所谓的“片状流”层内，这是一种薄层(颗粒直径为10-60)，在固定床上，沙粒的体积浓度从接近0.6的值下降一个数量级。在一个波周期内，该层的厚度变化，最大厚度随着峰值屏蔽应力的增加而增加。床面流层内悬浮泥沙浓度的变化与声学多普勒测速仪测量的轨道速度基本同步，具有典型的0-π/5的相位滞后。相反，悬浮泥沙浓度在几厘米及以上具有较大的相位滞后。在板状流的中部和上部，成功地测量了它们在最大值时间附近的颗粒速度。这些速度随高度从波边界层外速度的约50%增加到70%而微弱增加。与前人的实验结果进行了比较，发现在恒定单向流和振荡水洞(OWTs)中的观测结果基本一致，尽管悬沙浓度和总输沙率有明显的差异。实验结果还与两个截然不同的模型进行了比较：一个1DV悬浮模型用于边界粗糙度增强的振荡流，另一个模型用于定常单向流的两相碰撞颗粒流。虽然悬浮模型较好地描述了速度分布，碰撞模型较好地描述了浓度分布，但这两个模型都不能准确地预测整个片状水流的速度和浓度，从而也就是整个垂直范围内的泥沙通量。
For the first time, detailed measurements of sediment concentrations and grain velocities inside the sheet flow layer under prototype surface gravity waves have been carried out in combination with measurements of suspension processes above the sheet flow layer. Experiments were performed in a large-scale wave flume using natural sand. Sand transport under high waves in shallow water is mainly contained within the so-called “sheet flow layer,” a thin layer (10–60 grain diameters) in which the volume concentration of sand decreases by an order of magnitude from a value near 0.6 at the stationary bed. The thickness of the layer varies over a wave cycle and the maximum thickness increases with increasing peak Shields stress. The concentrations within the sheet flow layer vary approximately synchronously with the orbital velocity measured by an Acoustic Doppler Velocimeter (ADV) located 0.1 m above the bed, with typical phase lags of 0–π/5. In contrast, the suspended sediment concentrations a few centimeters and higher above the bed exhibit larger phase lags. Grain velocities were successfully measured in the middle and upper portions of the sheet flow layer around the time of their maximums. These velocities increased weakly with elevation from approximately 50% to 70% of the velocity outside the wave boundary layer. The observations are compared to previous experimental work and are found to be mainly consistent with observations in steady unidirectional flows and in oscillating water tunnels (OWTs), although differences in the suspended sediment concentration and the total sediment transport rate are apparent. Observations are also compared to two very different models: a 1DV suspension model for oscillatory flow with enhanced boundary roughness and a two-phase collisional grain flow model for steady unidirectional flow. While the suspension model describes the velocity profile fairly well and the collisional model describes the concentration profile well, neither model accurately predicts both the velocity and the concentration and therefore the sediment flux over the full vertical extent of the sheet flow.