网格尺寸及初始扰动对辫状河流演变模拟的影响

Influence of grid scale and initial disturbance on the process of numerical braid rivers

辫状河流是以沙洲与汊道的交错分布为主要特征的河网系统。采用水沙与河床耦合动力学二维数值模型,模拟了以推移质输移为主的辫状河流演变过程,探讨了网格尺寸及初始河床扰动对辫状河流演变的影响。结果表明,数值模型能较好模拟辫状河流的多种形态与动力学特征。首先,泥沙淤积形成沙洲并呈鱼鳞状分布于整个河道;随后,沙洲通过自身增长、分裂,并相互合并而发育;最后,沙洲形态保持相对稳定。当网格尺度大于某临界尺度时,网格尺度影响辫状指数和沙洲高度,尺度越大,辫状指数和沙洲高度越小,但不影响面积、周长、长轴和短轴长等沙洲形态特征。扰动类型及其量级是引起流场不均匀性从而触发沙洲形成的重要因素,但不影响辫状河流及其沙洲的形态特征。

Braid rivers are characterized by the staggered distribution of bars and channels. A 2 D numerical model is used to simulate the processes of braid rivers dominated by bed load transport. The influence of grid size and initial bed disturbance on braided river processes are also considered. Results show that the numerical model can be efficient in simulating the morphological and dynamic characteristics of braid rivers. Result shows that sediment deposits form bars in the shape of fish scales. Subsequently, the bar grows, splits, and merges with other bars, before the bar shape remains relatively stable. When the grid-scale is larger than a critical scale, the grid-scale affects the braiding index and the height of the bar, whereby, the larger the grid scale, the smaller the braiding index and the height bar. However, the grid scale doesn’t affect the morphological characteristics of the bar, which includes the area, perimeter, long axis length, and short-axis length. The disturbance type and its magnitude are the important factors which cause the flow field inhomogeneity and trigger the formation of bars, but do not affect the morphological characteristic of the bar.