Fluvial Dynamics of Large River Confluences

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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.)