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

实验研究了复合明渠凹区三维流动结构和泥沙沉积。采用PIV方法，以凹陷形状、凹陷层的相对水平和是否存在直堤为设计项，研究了它们对凹区流动结构的影响。此外，在同一实验室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。将凹陷层相对于主河道层的相对高度作为控制参数。在无过岸流的情况下，在凹区产生单一垂直涡。在河漫滩上有过岸流的情况下，在凹区横向涡与垂直涡共存。在矩形凹形情况下，随着孔洞深度的增加，横向涡的纵向尺度相对于垂直涡变得更大。因此，砂床附近的流态变得非常不同，这影响了砂的数量和形状。在梯形凹度中，斜台阶产生倾斜的横向涡。该涡旋的纵向尺度比矩形情况下的小，从而产生与正常阶梯情况不同的流型。因此，过岸水流在凹区产生横向涡，进而改变水流结构和沉积形态。其次，研究了河漫滩、中层河漫滩和主河道三断面河道凹区水流结构和悬沙输运。在三断面河道中，横向动量交换加速了流速，从而增加了冲积平原的输沙速率。凹区沉积物沉积速率受相对层位的影响较大。当凹陷层位与主通道层位相同时，床层荷载和悬空荷载均由主通道向凹陷输送。当凹面的河床高度高于主河道的河床高度时，冲积平原泥沙荷载由凹面向主河道输送。这些特征与凹凸带的三维流动结构密切相关。凹带植被阻止了主河道的流入，并改变了下坡带的再附着长度。这种影响使沙的沉积比没有植被的情况更平坦。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