River confluences - locations where rivers join one another - are fundamental components of natural drainage networks. Differences in topography, geology, soils, land use, and human activities within watersheds upstream of confluences can produce differences in thermal or chemical properties of river flows and in the materials transported by these flows. Mixing is initiated along the mixing interface (MI) that develops between the two incoming streams with different properties. Therefore, the understanding of fluvial processes at confluences is important for determining river mixing both at and downstream of individual confluences and at the scale of drainage networks.
The primary goal of this thesis is to describe the main mechanisms that control mixing and transport at river confluences and the role played by the complex flow structures in the flow and how they change with planform geometry and other flow and geometrical parameters. The study is carried out using Computational Fluid Dynamics modeling based on the state of the art Detached Eddy Simulation approach and High Performance Computing. By starting with a mixing layer between parallel streams with simple geometry, the model is validated based on laboratory experiment data. Moreover, some hypotheses regarding the growth of the mixing layer are amended with the extensive data provided by the model, which is a valuable supplement to the experiment. By performing a detailed parametric study in very long and wide domains for simplified cases one can focus on the spatial development of the MI and the large scale coherent structures forming within and in the vicinity of the MI without the complications of other factors. More specifically, the influence of velocity and density difference of the two streams, flow depth, inflow conditions and angles between the two streams on the spatial development of the MI is analyzed. The data resulting from these simulations conducted in simple geometries is a unique set of data which can be used to test and improve theoretical models used to predict global parameters describing flow and mixing at natural river confluences. In particular, this research uses for the first time well resolved Large Eddy Simulation based techniques to investigate how density differences between the incoming streams affect the spatial development of the mixing interface and mixing downstream of the confluence apex.
In order to investigate flow dynamics, mixing processes and effects of temperature stratifications at natural river confluences with discordant bed, a series of simulations is performed for the confluence of the Ebro and Segre Rivers in Spain, which is one of the most studied confluences in Europe. With the detailed survey data of the confluence bed and flow conditions data provided, the goal is to understand the main mechanisms responsible for mixing at a confluence with a large bed discordance and how the velocity ratio between the two incoming streams affects mixing. Besides, more insights are provided that if temperature stratification effects affect significantly flow structure and mixing based on real conditions recorded at a natural confluence. The study provides a comprehensive set of flow data in the confluence including velocity, temperature distribution etc. It serves as important supplement to the field measurements, which are generally more difficult to obtain. It also allows estimating scale effects between field conditions and conditions at which laboratory experiments of confluence flow and mixing are conducted.