Abstract Introduction of fine sand into gravel-bed rivers has surged tremendously as a result of human activities. The ecological concerns for river habitats have called for suitable management strategies for cleaning these particles from gravel bed rivers. This paper is concerned with determining movement threshold of fine sand initially entrapped in the spaces between large immobile elements resembling a coarse riverine substrate with flow depths comparable to non-erodible element sizes. Static equilibrium condition was reached during an experimental run by maintaining a zero-supply condition for sand. It was assumed that the flow condition corresponding to this sand level was representative of critical conditions for onset of sand motion. A nonlinear regression curve was fitted between the level of the sand and associated critical shear stress. The flow field measurements using an ultrasonic Doppler rofiler delineate main features of the shallow flow over large roughness elements such as sharp velocity gradients near the crest of the immobile elements, and also a deviation of instantaneous velocity probability density function from normal distribution very close to the sand level.

中文翻译：

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固定砾石基质风选细砂的实验观察

摘要 由于人类活动，细砂流入砾石层河流的数量急剧增加。河流栖息地的生态问题要求采取适当的管理策略来清除砾石床河流中的这些颗粒。本文涉及确定细砂的运动阈值，这些细砂最初被困在类似于粗河流基质的大型不动元件之间的空间中，其流动深度与不可侵蚀的元件尺寸相当。通过保持沙子的零供应条件，在实验运行期间达到了静态平衡条件。假定对应于该砂层的流动条件代表砂运动开始的临界条件。非线性回归曲线拟合在沙子的水平和相关的临界剪切应力之间。