Numerical simulation of bar and island morphodynamics in anabranching megarivers
Numerical simulation of bar and island morphodynamics in anabranching megarivers
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DOI：
10.1002/jgrf.20132
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发表时间：
2013-12
期刊：
影响因子：
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通讯作者：
Andrew Nicholas;Philip J. Ashworth;G. H. S. Smith;S. Sandbach
中科院分区：
文献类型：
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作者：
Andrew Nicholas;Philip J. Ashworth;G. H. S. Smith;S. Sandbach
利用一种新的数值模型，研究了数百年来世界上最大的支流河流中的沙洲和岛屿形态动力学，包括水动力学、泥沙运移、河岸侵蚀和洪泛平原的发展。将模拟河道形态与自然河流形态进行了比较，结果表明，无论是在河道宽度、深度和沙洲形状分布的统计特征方面，还是在单元沙洲、复合沙洲和岛屿演变机制方面，模拟河道形态都是真实的。结果表明，沙洲和岛屿的稳定性可能对水文状况很敏感，因为洪水强度的较大变化促进了紧急沙洲的形成，这些沙洲可以通过植被定殖来稳定。模拟说明了单元沙洲形成的一系列机制，这些机制与局部河床或河岸的不稳定性有关。这种联系可以解释在一些以稳定的植被岛屿和缓慢的河道变化速率为特征的大型再分支河流中观察到的单元沙洲频率降低的现象。模型结果表明，沙粒床的悬浮程度可能是沙洲形态动力学和河道网络演化的重要控制因素，因为它对沙粒输运方向有影响。因此，总沙负荷在河床和悬浮物之间的分配差异可能部分解释了世界上最大的砂床河流分支风格的差异。这些结果突出表明，为了支持形态动力学模型中沙质输运力学的参数化，需要来自大型河流的空间分布的水流和泥沙输运数据集。
Bar and island morphodynamics in the world's largest anabranching rivers are investigated using a new numerical model of hydrodynamics, sediment transport, bank erosion, and floodplain development, operating over periods of several hundred years. Simulated channel morphology is compared to that of natural rivers and shown to be realistic, both in terms of the statistical characteristics of channel width, depth, and bar shape distributions, and mechanisms of unit bar, compound bar, and island evolution. Results demonstrate that bar and island stability may be sensitive to hydrologic regime, because greater variability in flood magnitude encourages the formation of emergent bars that can be stabilized by vegetation colonization. Simulations illustrate a range of mechanisms of unit bar generation that are linked to local bed or bank instabilities. This link may explain the reduced frequency of unit bars observed in some large anabranching rivers that are characterized by stable vegetated islands and slow rates of channel change. Model results suggest that the degree to which sand‐sized bed material is carried in suspension likely represents an important control on bar morphodynamics and channel network evolution, because of its influence on sand transport direction. Consequently, differences in the partitioning of the total sand load between bed load and suspension may provide a partial explanation for contrasting styles of anabranching in the world's largest sand‐bed rivers. These results highlight a need for spatially‐distributed flow and sediment transport data sets from large rivers, in order to support improved parameterizations of sand transport mechanics in morphodynamic models.