Hydraulic characteristics and bed deformation on embayment and mid flood plain in compound open channels

通过实验研究了复合明渠凹陷区的三维水流结构和泥沙淤积。选取凹形、凹床相对高度和丁坝的存在作为设计项目，用PIV方法研究了它们对凹区水流结构的影响。在同一水槽中进行了泥沙淤积试验，考虑了水流结构与输沙过程的关系，以凹底相对于主槽底床的相对高度作为控制参数。在没有漫滩流的情况下，在凹区内产生单一的垂直涡流。在漫滩流情况下，凹区内横向涡与垂直涡共存。在矩形凹面情况下，随着圆锥…深度的增加，横向涡相对于垂直涡的纵向尺度变大更多的洞。结果，床层附近的流型变得非常不同，这影响了泥沙沉积的数量和形状。在梯形凹坑中，斜台阶产生倾斜的横向涡。这种旋涡的纵向尺度变得比矩形情况下的小，这就产生了与正常阶跃情况不同的流型。因此，漫滩水流在凹区产生横涡，从而改变水流结构和泥沙淤积形态。其次，本文研究了滩区、中滩和主槽三段河道凹区的水流结构和悬沙输移。在三段河道中，横向动量交换加速了滩地的流速，从而提高了滩地的输沙率。相对床面高度对凹坑内泥沙淤积速率有较大影响。当凹坑的河床水位与主槽河床相同时，推移质和悬移质将从主槽输送到凹坑中。当凹坑的河床水位高于主槽河床时，滩地泥沙将从凹坑向主槽输送。这些特征与凹陷区的三维流动结构密切相关，凹陷区的植被阻碍了主槽的流入，改变了下坡区的再附着长度。这种效应使沙粒沉积比没有植被的情况下更平坦。较少

We investigated three-dimensional flow structures and sand deposition in an concavity zone in a compound open channel experimentally. We picked up the concavity shape, the relative level of concavity bed and the existence of spur dikes as design items and investigated their effects on the flow structures in concavity zones by using PIV method. Furthermore, sediment deposition tests was conducted in the same laboratory flume and the relation of flow structures to sediment transport process was considered.The relative level of the concavity bed against the main channel bed was considered as a control parameter. A single vertical vortex is generated in a concavity zone in the case without overbank flow. In the case with overbank flow on flood plain, a transverse vortex coexists with the vertical vortex in the concavity zone. In rectangular concavity case, the longitudinal scale of the transverse vortex becomes larger relative to the vertical vortex with an increase of the depth of the con … More cavity. As a result, the flow pattern near the bed becomes very different from each other and this affects the amount and the shape of sand deposition. In trapezoidal concavity, skew step produces inclined transverse vortex. The longitudinal scale of this vortex becomes smaller than that in the rectangular case and this generates different flow patterns from the normal step case. Consequently, overbank flow causes transverse vortices in the concavity zone and then changes flow structures and sand deposition shape.Next, we investigated flow structures and suspended sediment transport in concavity zone on triple section channel with flood plain, mid-level flood plain and main channel. In the triple section channel, transverse momentum exchange accelerates the velocity and then increases sediment transport rate on the flood plain. The sediment deposition rate in the concavity is much affected by the relative bed level. When the bed level of the concavity is the same as the main-channel bed, the bed load and the suspended load are transported from the main channel into the concavity. When the bed level of the concavity is higher than the main-channel bed, the flood-plain sediment load is transported from the concavity to the main channel. These features are well related to the 3-d flow structures in the concavity zones.The vegetation in concavity zone prevents inflow from the main channel and changes reattachment length in down step zone. This effects makes sand deposition flatter than in the case without vegetation. Less