Study on a mechanism of sheet-flow sediment transport

近年来，人们对片流输沙的研究产生了浓厚的兴趣，因为在这种模式下，航道会发生严重的沙滩冲刷和淤积。低流体牵引力作用下的泥沙运动已有许多研究。相比之下，流体/泥沙混合物在高流体牵引力作用下的行为还知之甚少。在本研究中，利用高速摄像机测量了泥沙颗粒在振荡流作用下的运动，并对其特性进行了详细的分析。主要研究结果如下：(1)薄板流层厚度随屏蔽参数的增加几乎呈线性增加。同时，跃变层厚度并没有明显增加。(2)跃变层的含沙量剖面可用指数衰减曲线表示，而在片状流的含沙量剖面可用凸形曲线表示。(3)用两种方法测量了输沙率。一种是通过含沙量与输沙速度的乘积估计，另一种是直接跟踪视频帧中的一组绘制颗粒。结果表明，两种方法得到的输沙率相差不大。作为泥沙的颗粒材料之间的差异也不大。(4)将得到的泥沙输移率绘制在前人报道的散射体中心。计算结果与Madsen-Grant公式的预测结果吻合较好。(5)本研究虽然没有覆盖很大的试验范围，但所获得的结果为研究床面水流的力学和泥沙输移的定量描述提供了有用的信息。

Study on sheet-flow sediment transport has recently attracted much interest because severe beach erosions and shoalings of fairway occur under this mode. There have been many studies on sediment transport under low fluid tractive force. In comparison, the behavior of fluid/sediment mixture under high fluid tractive force is poorly understood. In this study, motions of sediment particles under oscillatory flows were measured with a high speed video camera, and the properties were carefully analyzed. The main results are as follows.(1) The thickness of the sheet flow layer increases almost linearly with the Shields parameter. Meanwhile the thickness of the saltation layer does not increase with it noticeably.(2) The profile of sediment concentration can be represented by an exponentially decay curve in the saltation layer, and an upward convex curve in the sheet-flow layer.(3) The sediment transport rate was measured by the following two ways. One is estimated by the product of sediment concentration and sediment transport velocity, the other is to direct-trace a group of painted particles in the video frame. As a result, the two methods yield little difference in the obtained sediment transport rate. Little difference is also found between particle materials used as the sediment.(4) The obtained sediment transport rates are plotted in the center of scatters reported by other researchers previously. The results also agree well with those predicted by Madsen-Grant formula.(5) Although this investigation does not cover a very wide experimental range, the above obtained results yield useful information on the mechanics of the sheet-flow and quantitative description of the sediment transport.