Fluvial Dynamics of Large River Confluences

汇合处是两条河流交汇的地方，是天然河网的基本组成部分，但大多数关于汇合处的水流、泥沙运动和河道变化的研究都集中在实验室汇合处和小的自然汇合处。对于大型河流汇流的小规模研究得出的结论的相关性知之甚少，迫切需要对大型河流汇流进行基于过程的实地调查，以探索汇流动力学如何随空间尺度变化，以及这种变化对水沙网络尺度运动的影响。本项目将探讨各种控制因素，包括汇流几何形状、上游支流动量比对大型河流汇流内水流结构和河道形态变化的影响。将在印第安纳州沃巴什河系统的三个汇合处收集有关河道形态、三维水流结构和河床地形的现场数据，这些汇合处呈现出不同的几何形状。数据收集活动将与位于厄巴纳的美国地质调查局地区总部合作进行，该总部拥有一整套用于大河研究的最先进仪器。每年将在每个汇合处收集一至两次活动的数据，为期三年。将选择一组采样事件来代表每个站点的一系列水文条件。数据分析将包括开发基于计算机的三维床形态空间格局、三维速度场和每个地点水温/混合效验图，以探索汇流平台、动量通量比和床形态对汇流动力学的影响。这一分析将包括记录由流入流动量通量比的变化引起的合流床形态的变化，以及流动结构和混合对这些变化的响应。通过与英国利兹大学（University of Leeds）和谢菲尔德大学（University of Sheffield）的研究团队的持续合作，现场数据将被用作先进的三维流量数值模型的输入，以测试该模型模拟自然大河汇合处流量的能力。数值模拟将补充实地调查，允许在理论范围内更全面地了解特定地点的调查结果，并为解释一般过程在特定地点的情况下如何表现不同提供基础。除了对大河汇合处的动力学以及汇合处通过河网调节水沙通量的作用提供基本见解外，研究结果还有助于提高对与汇合处的沉积/侵蚀有关的实际问题的理解和管理，例如河流航行，通过详细定义大河汇合处的自然生境条件，加强河流地貌与生态之间的联系。以及涉及河流系统中污染物和其他溶解或悬浮成分扩散的应用问题。这项研究将直接与大学和K-12教育挂钩，并通过伊利诺斯大学新成立的水作为复杂环境系统中心进行推广。与美国地质调查局的合作关系是该项目的关键组成部分，通过这种合作关系，研究结果有真正的机会融入更广泛的项目和国家利益的活动中。（这一行动是一项机构间协议，旨在支持美国地质勘探局在实地收集数据方面的技术援助。）

Confluences, the locations where two rivers join, are fundamental components of natural river networks, yet most studies of flow, sediment movement and channel change at confluences have focused on laboratory confluences and on small natural confluences. Little is know about the relevance of conclusions derived from small-scale studies for large-river confluences and a critical need exists for process-based field investigations of large-river confluences to explore how confluence dynamics change with spatial scale and implications of such changes for network-scale movement of water and sediment. This project will explore the influence of various controlling factors, including confluence geometry, momentum ratio of the upstream tributaries, on flow structure and changes in channel form within large river confluences. Field data on channel form, three-dimensional flow structure and bed topography will be collected at three confluences along the Wabash River system in Indiana that exhibit different geometries. Data-collection activities will be conducted in collaboration with the District Headquarters of the U.S. Geological Survey in Urbana, which maintains a full suite of state-of-the-art instrumentation for large-river research. Data will be collected for one to two events per year at each confluence over a period of three years. The suite of sampled events will be chosen to represent a range of hydrological conditions at each site. Data analysis will involve the development of three-dimensional computer-based renderings of spatial patterns of bed morphology, 3-D velocity fields and water temperature/mixing at each site to explore the influences of confluence planform, momentum-flux ratio and bed morphology on the dynamics of the confluences. This analysis will include documentation of changes in confluence bed morphology resulting from changes in momentum-flux ratio of incoming flows, and the response of flow structure and mixing to these changes. Through an ongoing collaboration with research teams at the University of Leeds and the University of Sheffield in the UK, the field data will be used as inputs to an advanced numerical model of three-dimensional flow to test the capability of the model to simulate flow through natural large-river confluences. The numerical modeling will complement the field investigation by allowing findings for site-specific locations to be understood more completely within a theoretical context and by providing the basis for explaining how general processes manifest themselves differently in site-specific contexts. Besides providing basic insights into the dynamics of large river confluences and the role of confluences in regulating fluxes of water and sediment through river networks, results of the research contribute to improved understanding and management of practical issues related to sedimentation/erosion at confluences, such as river navigation, enhanced linkages between river geomorphology and ecology by defining in detail physical habitat conditions at large-river confluences, and applied problems involving the dispersal of contaminants and other dissolved or suspended constituents within river systems. The research will be tied directly to university and K-12 education and outreach through the newly established Center for Water as a Complex Environmental System at the University of Illinois. A partnership with the U.S. Geological Survey is a critical component of the project and, through this partnership, real opportunity exists for results of the research to become integrated into broader programs and activities of national interest. (This action is an outgoing interagency agreement to support the USGS's technical assistance in the collection of data in the field.)