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Numerical investigation of avulsions in gravel‐bed braided rivers
Hydrological Processes ( IF 3.2 ) Pub Date : 2020-06-26 , DOI: 10.1002/hyp.13837 Haiyan Yang 1
Hydrological Processes ( IF 3.2 ) Pub Date : 2020-06-26 , DOI: 10.1002/hyp.13837 Haiyan Yang 1
Affiliation
Gravel‐bed braided rivers are highly energetic fluvial systems characterized by frequent in‐channel avulsions, which govern the morphodynamics of such rivers and are essential for them to maintain a braided planform. However, the avulsion mechanisms within natural braided rivers remain unclear due to their complicated hydraulic and morphodynamic processes. Influenced by neighbouring channels, avulsions in braided rivers may differ from those of bifurcations in single‐thread rivers, suggesting that avulsions should be studied within the context of the entire braid network. In this study, braiding evolution processes in gravel‐bed rivers were simulated using a physics‐based numerical model that considers graded bed‐load transport by dividing sediment particles into multiple size fractions and vertical sediment sorting by dividing the riverbed into several vertical layers. The numerical model successfully produced braiding processes and avulsion activities similar to those observed in a laboratory river. Results show that bend evolution of the main channel was the fundamental process controlling the occurrence of avulsions in the numerical model, with a cyclic process of channel meandering by lateral migration that transitioned to a straight channel pattern by avulsion. The radius of bend curvature for triggering avulsions in the numerical model was measured and it was found that the highest probability for a channel bend to generate an avulsion occurs when its radius of curvature is approximately 2.0–3.3 times the average anabranch width. Other types of avulsion were also observed that did not occur specifically at meander bends, but upstream meander evolution indirectly influenced such avulsions by altering channel pattern and discharge to those locations. This study explored the processes and mechanisms of several types of avulsion, and proposed factors controlling their occurrence, namely increasing channel curvature, high shear stress, tributary discharge, riverbed gradient and upstream channel pattern, with high shear stress being a direct indicator. Furthermore, avulsions in a typical gravel‐bed braided river, the Waimakariri River in New Zealand, were analysed using sequential Google Earth maps, which confirmed the conclusions derived from the numerical simulation.
中文翻译:
砾石层辫状河流撕裂的数值研究
砾石辫状河流是高能的河流系统,其特征是频繁的河道内撕脱,支配着这些河流的形态动力学,对于保持辫状的平面形态至关重要。但是,由于天然辫状河的水力和形态动力学过程复杂,其撕脱机制仍不清楚。受邻近渠道的影响,辫状河的撕脱可能与单线河流的分叉不同,这表明应在整个辫状网络的背景下研究撕裂。在这个研究中,使用基于物理学的数值模型模拟砾石河道的辫状演化过程,该模型考虑了通过将沉积物颗粒划分为多个大小部分并通过将河床划分为几个垂直层的垂直沉积物分选来进行分级的床荷运输。该数值模型成功地产生了与在实验室河流中观察到的相似的编织过程和撕脱活动。结果表明,在数值模型中,主河道的弯折演化是控制撕脱发生的基本过程,横向偏移使河道蜿蜒的循环过程通过撕脱转变成笔直的河道型式。在数值模型中测量了触发撕脱的弯曲曲率半径,发现当通道弯曲的曲率半径约为平均分支宽度的2.0-3.3倍时,发生通道弯曲的可能性最高。还观察到了其他类型的撕裂,这些弯曲在弯弯曲曲时没有特别发生,但是上游弯弯曲曲通过改变通道模式和向这些位置的排放而间接影响了这种撕裂。这项研究探索了几种撕脱的过程和机理,并提出了控制它们发生的因素,即增加河道曲率,高剪切应力,支流流量,河床坡度和上游河道形态,而高剪切应力是直接指示。此外,
更新日期:2020-06-26
中文翻译:
砾石层辫状河流撕裂的数值研究
砾石辫状河流是高能的河流系统,其特征是频繁的河道内撕脱,支配着这些河流的形态动力学,对于保持辫状的平面形态至关重要。但是,由于天然辫状河的水力和形态动力学过程复杂,其撕脱机制仍不清楚。受邻近渠道的影响,辫状河的撕脱可能与单线河流的分叉不同,这表明应在整个辫状网络的背景下研究撕裂。在这个研究中,使用基于物理学的数值模型模拟砾石河道的辫状演化过程,该模型考虑了通过将沉积物颗粒划分为多个大小部分并通过将河床划分为几个垂直层的垂直沉积物分选来进行分级的床荷运输。该数值模型成功地产生了与在实验室河流中观察到的相似的编织过程和撕脱活动。结果表明,在数值模型中,主河道的弯折演化是控制撕脱发生的基本过程,横向偏移使河道蜿蜒的循环过程通过撕脱转变成笔直的河道型式。在数值模型中测量了触发撕脱的弯曲曲率半径,发现当通道弯曲的曲率半径约为平均分支宽度的2.0-3.3倍时,发生通道弯曲的可能性最高。还观察到了其他类型的撕裂,这些弯曲在弯弯曲曲时没有特别发生,但是上游弯弯曲曲通过改变通道模式和向这些位置的排放而间接影响了这种撕裂。这项研究探索了几种撕脱的过程和机理,并提出了控制它们发生的因素,即增加河道曲率,高剪切应力,支流流量,河床坡度和上游河道形态,而高剪切应力是直接指示。此外,