Flow and Suspended Sediment in Compound Channels with Trees in the Floodplain

当水面高度超过漫滩高度时，复合河道中的水流具有额外的剪切和三维流动特征。悬浮沉积物的沉积在河道规划中起着至关重要的作用，而洪泛区的树木则进一步增加了复杂性。在河漫滩上进行了一种复合圆柱形河道中悬浮泥沙沉积的实验研究。圆柱的排列有三种方式。当柱阵较低时，平流对浮沙在漫滩上的输运有影响。随着柱体在漫滩上的覆盖比的增加，漫滩上的流速变低，扩散作用变大。确定了最大有效沉积速率，得到了其与速度场和湍流相干结构的关系。越岸水流阻力可分解为表面阻力和形态阻力。漫滩上的树木造成的形式阻力，可以明确地表示为树干的形式阻力和由枝叶复合物产生的宏观湍流造成的动量损失。公式中的水动力参数是通过对明渠流动中相干结构的现有知识的批判性审查来确定的。在不调整系数的情况下，所提出的公式能很好地估计通过洪道的流量。建立了复合通道流的应力湍流代数模型。校正后的系数表明，湍流的正态分量在达到最佳值时被低估了。采用新系数，该模型成功地再现了一次速度场和二次速度场以及雷诺应力的主要分量。阐明了由二次流引起的平流在紊流扩散的横向动量输运中起主要作用。

Flow in compound channels experiences additional shear and three-dimensional flow features when the water surface level exceeds the level of floodplain. Deposition of suspended sediment imposes crucial role in the planning of river courses and trees in the floodplain add further complexity.Deposition of suspended sediment was experimentally studied in a compound channel with circular cylinders in the floodplain. The arrangement of cylinders was done in three ways. When the array of cylinders is low, the advection plays influential role in the transport of suspended sediment over the floodplain. As the ratio of the coverage of cylinders in the floodplain increases, the velocity over the floodplain becomes low and the diffusive action becomes substantial. The maximum effective deposition rate was determined and the relation with the velocity field and coherent structure of turbulence was obtained.Resistance of overbank flow is decomposed into surface resistance and form resistance. Form resistance, caused by trees in the floodplain, is explicitly formulated by the form drag of trunks and momentum loss due to macroturbulence generated by a branch-leaf complex. Hydrodynamic parameters in a formula are determined by the critical review of existing knowledge on coherent structures in open channel flows. Without any tuning of coefficients the proposed formula gives good estimation of discharge through floodways.An algebraic stress turbulence model was developed for a compound channel flow. The calibration of coefficient showed the the normal component of turbulence is under-estimated when the best value for a duct flow. With a new coefficient the model succeeded in reproducing the primary and secondary velocity fields, and major component of the Reynolds stress. It is elucidated the the major role is played by the advection due to secondary flow for the transverse momentum transport over the turbulent diffusion.