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

当水面线超过滩地线时，复式河槽水流将出现附加剪切和三维水流特征。悬移质泥沙的淤积是河道规划中的一个重要问题，而滩地树木的存在又增加了河道规划的复杂性。气缸的排列有三种方式。当圆柱排列较小时，平流对悬沙的输移起着重要的作用。随着漫滩中圆柱覆盖率的增加，漫滩上的流速变低，扩散作用变大。确定了最大有效淤积率，得到了最大有效淤积率与流速场和湍流拟序结构的关系，将漫滩水流阻力分解为面阻力和形阻力。形式阻力，在河漫滩树木造成的，明确制定的形式阻力的树干和动量损失，由于宏观湍流产生的枝叶复合体。公式中的水动力参数是通过对明渠水流拟序结构的现有知识进行严格审查来确定的。在不调整系数的情况下，该公式能较好地估算出泄洪道的流量。对系数的标定表明，在管道流动的最佳值时，湍流的法向分量被低估了。采用新的系数，该模型成功地再现了一次和二次速度场，以及雷诺应力的主要分量。阐明了二次流引起的平流对湍流扩散过程中动量横向输运的主要作用。

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.