Resolving two-dimensional flow structure in rivers using large-scale particle image velocimetry: An example from a stream confluence

Large-scale particle image velocimetry (LSPIV) has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems. LSPIV has typically been used in the field to obtain velocity or discharge measurements in relatively simple one-dimensional flow. Detailed two-dimensional or three-dimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling PIV limiting factors such as poor particle seeding, the need for camera rectification, and challenging field conditions. In this study we implement a low-cost LSPIV setup using a high-resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch. Time-averaged 2-D velocities derived from LSPIV are compared with those measured with an acoustic Doppler velocimeter (ADV) in the camera's field of view. We also assess the capabilities of this setup to resolve turbulent and time-averaged flow structures at a stream confluence. Our results reveal that even in challenging field conditions a basic LSPIV setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers. The resulting dataset contains velocity information at high spatial and temporal resolution, a significant advance in understanding flow processes at stream confluences. Our LSPIV analysis provides support for previous numerical modeling studies that have distinguished between Kelvin-Helmholtz and wake modes of turbulent behavior within the mixing interface at confluences. This study shows that LSPIV can provide unprecedented levels of resolution of surface velocity patterns on rivers. Detailed velocity data derived from LSPIV can be used to evaluate numerical predictions of flow structure in complex fluvial environments.