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Flow Structure at Three Small Confluences during Transport-Effective Flow Stages

Authors: Tanya Shukla*, Geography, University Of Illinois, Urbana Champaign, Quinn Lewis, Department of Earth & Atmospheric Science, Indiana University Bloomington, Bruce Rhoads, Geography, University Of Illinois, Urbana Champaign
Topics: Geomorphology
Keywords: fluvial geomorphology, three-dimensional flow, confluences
Session Type: Paper
Presentation File: No File Uploaded

The combining of flows from separate tributaries at confluences results in complex hydrodynamic conditions that play an important role in confluence morphodynamics. The hydrodynamic conditions are characterized by highly three-dimensional patterns of fluid motion, controlled mainly by the confluence planform and momentum ratio of the incoming flows. Despite abundant previous work, understanding of 3D flow structure at confluences remain poorly constrained, preventing the development of a universal conceptual model. The purpose of this study is to analyze 3D flow structure at three confluences with different planform configurations at relatively high stages. Previous work at these confluences has examined flow structure at low stages that were not capable of mobilizing bed material, and thus did not directly influence morphodynamics. Measurements of three-dimensional velocities were obtained along several cross-sections at each confluence using a Sontek M9 acoustic Doppler current profiler and were analyzed using the Velocity Mapping Toolbox (VMT) (Parsons et al., 2013). The structure of depth-averaged and cross-sectional downstream velocities reveals the presence of a stagnation zone near the upstream junction corner, the size of which depends upon the momentum ratio. Further, localized helicity around parcels of low streamwise velocity were observed, while dual surface converging helical cells as has been reported in earlier studies were not prominent. The 3D velocity measurements indicate that flow patterns for these high flows do not necessarily conform to patterns documented during low flows. This study contributes to the understanding of confluence hydrodynamics and changes in these hydrodynamics with changes in flow stage.

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