Flow dynamics at a river confluence on Mississippi River: field measurement and large eddy simulation

Trung Le, Postdoctoral Associate, St. Anthony Falls Laboratory, University of Minnesota

The mixing process of two branches in a river confluence is highly dynamic and can exhibit two flow modes depending on the momentum ratio: i) Kelvin-Helmholtz mode and ii) wake mode. In this work, we study the dynamics of a river confluence on Mississippi River branch in the city of Minneapolis, Minnesota, United States. Field measurements by Acoustic Doppler Current Profiler using on-board GPS tracking were carried out for five campaigns in the summer of 2014 and 2015 to collect both river bed elevation data and flow fields. High fidelity simulation (Large Eddy Simulation) is carried out to simulate the flow field with the total of 100 million grid points for the domain length of 3.2 km. The simulation results agree well with field measurements at measured cross-sections and reflect a complex interaction between two upstream branches at the confluence. The results show the existence of wake mode on the mixing interface of two branches near the upstream junction corner. The mutual interaction between the shear layers emanating from the river banks leads to the formation of large-scale energetic structures. The existence of these structures induce the emergence of  "switching mode", which enhances the lateral transfer of momentum and promote mixing. Our result here is a feasibility study for the use of eddy-resolving simulations in predicting complex flow dynamics in medium-size natural rivers.