Large Scale Particle Image Velocimetry Reveals Complex Two-Dimensional Structure of Flow at a Stream Confluence

Abstract:

River confluences are locations in drainage networks where two rivers converge, interact, and mix together. Confluences are often considered some of the most complex hydrodynamic environments in river systems. The interaction of confluent flows results in a variety of hydrodynamic zones, or discrete regions of the confluence characterized by distinct patterns of mean and turbulent flow. Capturing this hydrodynamic complexity in the field is difficult, but recent advances using Large-Scale Particle Image Velocimetry now provide a basis for exploring how patterns of flow at confluences vary over time and space at the water surface. This study uses LSPIV to examine 2D flow structure at a small confluence in Illinois. Results show that a large zone of flow stagnation develops at the upstream junction corner of this confluence and that a zone of flow separation from the channel banks occurs at the downstream junction corner. Shear layers characterized by vortices with vertical axes bound these two regions of low velocity. Interaction between the two confluent flows varies substantially over time, with the position of the mixing interface shifting laterally within the confluence as the flows compete for space. Irregular pulsing of tributary flows influences the extent of flow separation, distorts the stagnation zone, and results in temporal variations in the momentum fluxes of the two flows. Although the relative spatial arrangement of hydrodynamic features remains consistent, the absolute size and positions of these features varies over time in relation to dynamic interaction among the features. This research demonstrates the value of LSPIV for exploring spatial and temporal variation in confluence hydrodynamics at high levels of temporal and spatial resolution.