Causes of rapid mixing at a junction of two large rivers: Río Paraná and Río Paraguay, Argentina
Causes of rapid mixing at a junction of two large rivers: Río Paraná and Río Paraguay, Argentina
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DOI：
10.1029/2006jf000745
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发表时间：
2008-06
影响因子：
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通讯作者：
S. Lane;D. Parsons;J. Best;O. Orfeo;R. Kostaschuk;R. Hardy
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文献类型：
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作者：
S. Lane;D. Parsons;J. Best;O. Orfeo;R. Kostaschuk;R. Hardy
[1]航空和卫星观测表明，当大型河流汇合时，它们可能需要数百公里才能完全混合，但有时可能会非常迅速地混合。应用已建立的半理论分析表明，长的混合长度应预期。在两种情况下首次测量了一个大型河流交汇处（阿根廷巴拉那河和巴拉圭河，总宽度为2.8 km）的混合过程：第一次是在>400 km处混合，第二次是在交汇处下游仅8 km处混合完成。对于混合速度较慢的情况下，在一个点的调查表明，与近垂直剪切层的湍流剪切驱动的混合被限制在接近交界处（下游的postconfluence宽度的0.272倍）。样带调查显示，渗透更浑浊的水从里约巴拉圭下面的里约巴拉那，但这是不够的，以促进更快的混合。有没有明确的渠道规模的流通存在和缓慢的混合加剧了反向地形强迫的主流里约巴拉那河一侧。这使得巴拉圭河一侧的河水更加浑浊，靠近河床。在快速混合的情况下，我们发现了明确的渠道尺度环流。结合流之间的动量比加强了汇流处支流之间床高不一致的影响，并允许更浑浊的Rio Paraguay水进一步渗透到更深处的通道宽度内。动量比和河床形态之间的相互作用的重要性，在通道交界处的混合率取决于流域尺度的水文响应，这是更有可能不同的大汇流河流比小河流，由于不同的气候/地形区，他们可能会捕获。
[1] Airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but, on occasion, may mix very rapidly. Application of established semitheoretical analyses shows that long mixing lengths should be expected. The first measurements of mixing processes at a large river junction (Rio Parana and Rio Paraguay, Argentina, combined width ∼2.8 km) are presented at two occasions: first when they mix in >400 km, and second when mixing is complete in only 8 km downstream of the junction. For the case of slower mixing, at-a-point surveys showed that mixing driven by turbulent shear associated with a near-vertical shear layer was restricted to close to the junction (to 0.272 multiples of the postconfluence width downstream). Transect surveys showed penetration of more turbid water from the Rio Paraguay underneath the Rio Parana, but this was insufficient to promote more rapid mixing. There was no clear channel-scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream Rio Parana side of the river. This kept more turbid water on the Rio Paraguay side of the river, close to the bed. In the case of rapid mixing, we found clear channel-scale circulation. The momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid Rio Paraguay water further across the channel width deeper within the flow. The importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin-scale hydrological response, which is more likely to differ between large confluent rivers than small rivers, as a result of the different climatic/topographic zones that they may capture.