辫状河流不仅地貌形态复杂，动力特性显著，而且对人类活动等外部因素展现出高度敏感性。尽管雅江中游是西藏的经济、文化、社会中心，但由于高寒缺氧、交通不便、量测条件限制等原因，以往对此间辫状河流的研究并不深入。雅江的辫状河段不仅辫状强度高，有着独特的山谷约束，而且随着人工造林、水电资源开发等活动逐渐开展，人们对其水沙动力学过程与地貌演变趋势的认识需求日益加强。本文以雅江典型宽谷辫状河段为研究对象，就其平面形态的量化描述、动力学过程、植被与河流地貌的相互作用等三方面开展了深入研究与探讨。泥沙粒径分析结果表明研究河段的泥沙主要由细、中砂组成，为沙质河床。依据山谷约束与河流的平面形态，藕节状河谷单元内的辫状河流被划分为上游过渡段、中游发展段和下游收缩段。遥感影像的分析结果指出其辫状指数、洲滩数量等均呈现显著的季节性变化，并揭示了辫状汊道的宽度遵循等比例递减的分级规律。这一规律可以作为传统汊道辫状指数在量化描述辫状强度方面的补充，其分级系数的季节性变化规律可视为宽谷辫状河段的典型特征。构建了针对实际大型辫状河流基于物理学过程的数学模型，结合动态地形加速策略和分区并行计算方法，成功模拟了大型辫状河流的形成和演变过程。基于该模型研究了宽谷辫状河段的形态动力学特征，展现了辫状地貌特征参数在洪水过程中的快速响应特性，解释了二次流在辫状河型的形成和维持中起到的关键作用。通过模型研究了辫状汊道的输沙特性，发现了输沙汊道数量仅占总汊道数量的25%至50%。研究还发现了宽谷河段内存在的活跃辫状廊道，并揭示了山谷基岩约束和河段宽度对其形成起到的重要作用。基于GEE云平台开发了一种自动检测植被动态与河流演变的无监督算法，对过去三十五年人工林对辫状河貌的影响进行了定量分析。结果显示，部分原本分布零散且尺寸较小的辫状洲滩受沿岸人工林增长的影响，结构开始调整并形成了更稳定的大型洲滩。这导致了河段辫状强度开始降低，辫状廊道的活跃宽度随之减小。另一方面，河段内植被覆盖面积占比不足10%，并且由于环境因素制约，植被在年际间存在损耗现象，研究河段目前在整体上仍呈现活跃的辫状河流地貌演变特征。高原流域的造林工程亟需开展更深入的研究和更为精细化的管理。
Braided rivers not only possess complex geomorphic forms and significant dynamic characteristics, but also show a high sensitivity to external factors like human activities. Although the middle reaches of the Yarlung Tsangpo River are the economic, cultural, and social centers of Tibet, previous studies on its braided reaches were not in-depth due to factors such as high-altitude anoxia, inconvenient transportation, and measurement limitations. The braided reaches of the Yarlung Tsangpo River not only have a high braiding intensity but also unique valley constraints, and human activities such as afforestation and hydropower development are gradually being carried out. There is a growing need to understand its hydro-sediment dynamic processes and geomorphologic evolution. Focusing on a typical wide-valley braided reach of the Yarlung Tsangpo River, this study delves deep into its planform morphology, dynamic processes, and the interplay between vegetation and riverine landscapes.The results of grain size analysis show that the sediments in the study reach are mainly composed of fine and medium sand, indicating a sandy riverbed. Considering the valley constraints and the river‘s planform, braided streams within the nodal-shaped valley are categorized into upstream transition, midstream development, and downstream convergence reaches. Analysis of remote sensing images indicates significant seasonal variations in the braiding index and the number of bars, and reveals that the width of braided channels follows a proportionally decreasing hierarchical rule. The width hierarchical rule can serve as a supplement to the traditional braiding index in quantitatively describing braiding intensity. The hierarchical coefficient‘s seasonal variation is seen as a typical feature of braided reaches in the wide valley.Using physics-based equations, morphological acceleration strategies, and partitioned parallel computing methods, a mathematical model for real-world large braided rivers was constructed, successfully simulating the formation and evolution of large braided rivers in nature. With this model, the morphodynamic attributes of the wide-valley braided reach were examined, highlighting the quick response traits of braided geomorphic parameters amidst flooding, and elucidating the pivotal function of secondary flows in the genesis and sustenance of braided river morphologies. The model was used to study the sediment transport characteristics of channels, and it was found that the number of channels transporting sediment was only 25 to 50 percent of the total channels. The study identified active braided corridors within the wide valley and revealed the significant roles of valley bedrock constraints and variations in valley width in their formation.An unsupervised algorithm for automatically detecting vegetation dynamics and river evolution was developed based on the GEE cloud platform. A quantitative analysis was conducted on the impact of afforestation on braided river morphology over the past thirty-five years. The results show that some originally scattered and small-sized braided bars, influenced by the growth of artificial forests along the riverbank, began to adjust their structure and formed more stable and larger bars. This led to a reduction in the braiding intensity of the reach and a subsequent decrease in the active width of the braided corridor. However, the proportion of vegetation cover within the reach is less than 10%, and due to environmental constraints, there has been a phenomenon of vegetation loss over the years. The study reach still shows active braided evolution characteristics as a whole. Afforestation projects in plateau basins urgently need more in-depth research and more refined management.
