Flow evolution near the apex of two small stream confluences using large-scale particle image velocimetry

This study examines flow near the apex of two confluences with different geometries and hydrologic conditions. Characterization of flow at one of the confluences is based on Large Scale Particle Image Velocimetry (LSPIV) and Acoustic Doppler Velocimeter (ADV) measurements, whereas characterization at the other is based on LSPIV measurements only. At the first confluence, LSPIV and ADV measurements clearly show a zone of flow stagnation near the junction apex and a region of velocity deficit extending downstream from this stagnation zone into the center of the confluence. LSPIV reveals that shear layers exist on both sides of the stagnation zone and velocity-deficit region. Flow from the main channel moves slowly across the stagnation zone and is entrained into the tributary-side shear layer. LSPIV measurements at the second confluence indicate the lack of well-developed flow stagnation near the junction apex; instead, strong shear between the confluent flows generates well-defined 2D vortices. The results of this field study generally support hypotheses emerging from numerical simulations about the structure of flow near the apex region of confluences with different hydrological conditions.