Flow structures in actual rivers obtained by areal measurement of flow velocity and water surface elevation

Flow velocity and water surface elevation (WSE) are fundamental for understanding hydraulic phenomena in river engineering. Although underwater flow properties are not directly observable, these two parameters encapsulate the hydraulic properties governing river flow, as described by the conservation of mass and momentum equations. This information enables the understanding of actual hydraulics and facilitates the creation of digital twins, even during large scale flood events.

To measure flow velocity from UAV imagery, we developed a novel, reference-free image analysis method based on image conversion. This method eliminates the need for physical reference points, addressing practical challenges in field deployments. It leverages readily available camera information, including position (x, y, z) and orientation (pitch, roll, yaw). Complementary WSE data, obtainable from various sources, completes the required input. This allows accurate conversion of video pixel data to surface coordinates, enabling velocity measurements at any point within the river flow. Particle image velocimetry (PIV) is then applied to the converted images to derive the velocity field.

For WSE determination, we explored three approaches: Light Detection and Ranging (LiDAR), Structure from Motion (SfM), and edge-based downscaling of SfM. LiDAR data, while valuable and easy to observing, exhibits lower point density on the water surface compared to the surrounding non-water areas, depending on water surface conditions. However, even sparse LiDAR data in the mid-channel provides crucial hydraulic information. For SfM, we employed multiple UAVs capturing images at appropriate timing to resolve temporal WSE changes. As a downscaling approach using a single UAV, WSE data extracted solely from the riverbank can also be utilized.

We have begun accumulating observations of large-scale flow phenomena. Our results reveal cellular secondary currents and flow patterns over bedforms. Observations of cellular secondary currents show boiling-type phenomena occurring on the order of seconds, and more persistent cellular structures when averaging the flow field over one minute. From an engineering perspective, although these events are infrequent, they can significantly impact float-based discharge measurements when they occur. Observations of flow over bedforms show spatial variations in velocity and WSE along the flow direction, exhibiting wave-like patterns. The out-of-phase relationship between these wave patterns suggests they are associated with micro-bedforms, indicating active sediment transport. Furthermore, this understanding of sediment hydraulics can be used to estimate water depth.